Method for increasing the image rate of a sonar and sonar for the implementation of this method

ABSTRACT

The invention relates to a simple, cheap sonar system with high image rate, for the detection of objects and the imaging of sea bottoms. It consists in transmitting n uncorrelated successive codes in a sector of angular width exactly equal to n times the angular width of the reception sector θ R , the reception antenna continuing to turn during this time whilst the first signal transmitted has not yet reached the maximum range dmax, and in receiving, in the sector of angular width θ R , the echoes of these n codes, coming from n propagation regions which are adjacent in relation to the reception axis, and lying between 0 and dmax in space, each of them having a depth equal to dmax/n.

The present invention relates to a sonar system for the detection ofobjects and the imaging of sea bottoms. Such a sonar is intended to beinstalled in an underwater vehicle or in a ship's hull, at low cost; itmust therefore be simple and cheap, and have a good image rate. In fact,because of the low speed of propagation of sound in water, the rate ofrenewal of information must be high so as to carry out correct samplingof the terrain or to carry out automatic extraction of targets, whichextraction calls for the greatest possible independent returns from thetarget.

Classically, there exist three major types of operation for sonarsystems, and each leads to specific hardware.

In a pulse-action single-beam sonar with rotatable mechanical scanning,the antenna is a single-element one and a single electronic receptionsystem is required. This sonar is therefore simple and cheap. On theother hand, the transmission and reception transducers are generallydirectional and the reception transducer must remain aimed in thedirection of transmission so long as the signal likely to return fromthe maximum distance has not arrived at the antenna. The speed ofrotation of the antenna then remains limited to low values, and theimage rate, in particular for systems having good angular resolution, isvery low.

In a sonar with preformed channels, a wide sector is filled with soundon transmission, and at reception, with each pulse, channels are formedelectronically throughout the sound-filled sector. The image of acomplete sector is then obtained at a high image rate. This principle isvery powerful but the complexity of the hardware is great, especiallyfor installation in a small underwater vehicle.

Finally, in a continuous transmission frequency modulation (CTFM) sonar,the image rate is high but the resolution in distance is often low sinceit is inversely proportional to the number of spectral analysis filterspresent in the reception system. Moreover, the low reception band, afterspectral analysis, renders the targets fluctuating, this beingprejudicial to a good probability of detection.

In order to overcome these disadvantages and obtain a sonar which issimple, cheap, has wide-sector transmission, and offers an image rateincreased by a factor n, the invention proposes another method, to beimplemented in a sonar having a maximum range dmax and comprising amovable transmission antenna covering a current sector of angular widthθ_(E) and a movable reception antenna covering a current sector ofangular widthθ_(R) centred in relation to an axis turning like thereception antenna, called the reception antenna axis, characterized inthat it consists:

in transmitting, in the form of pulses, n uncorrelated successive codes(C1 to Cn) in a current sector of angular width θ_(E) exactly equal tonθ_(R), the transmission and reception antennae continuing to turnduring this time in such a way that the rate of transmission of thecodes corresponds to the time taken by the reception antenna to passfrom the current sector of angular width θ_(R) to a following sector ofangular width θ_(R), in the direction of turning of the receptionantenna;

and in receiving, in the current sector of angular width θ_(R), theechoes of these n codes, coming from n propagation regions which areadjacent in relation to the reception antenna axis, and lying between 0and dmax in space, each of them having a depth equal to dmax/n.

The subject of the invention is also a sonar for the implementation ofthis method.

Other features and advantages of the invention will emerge clearly inthe following description given by way of non-limiting example and madewith reference to the attached figures which represent:

FIG. 1: The region of terrain observed on transmission and on reception,for a single-beam sonar with rotatable mechanical scanning according tothe prior art,

FIG. 2: The chart of the times of the transmission pulses and theposition of the antennae as a function of time for a pulse-actionsingle-beam sonar with rotatable mechanical scanning according to theinvention,

FIG. 3: The region of terrain observed by a pulse-action single-beamsonar with rotatable mechanical scanning for progressive receptionantenna positions, according to the invention,

FIG. 4: The region of terrain observed by a side-scan sonar withparallel preformed channels, according to the invention,

FIG. 5: An example of the distribution of transmission frequencies in apulse-action single-beam sonar with rotatable mechanical scanningaccording to the invention,

FIG. 6: The general overview of a pulse-action single-beam sonar withrotatable mechanical scanning, according to the invention.

FIG. 1 represents the region of terrain observed on transmission and onreception for a single-beam sonar with rotatable mechanical scanning ofthe prior art, comprising a transmission antenna, 20, covering anangular sector θ_(E), and a reception antenna, 30, covering an angularsector θ_(R) such that θ_(E) is greater than θ_(R). In the figure, thepositions of the transmission, 20, and reception, 30, antennae arerepresented at the instant of transmission and at the end of receptionrespectively. Between transmission and reception, the reception antenna,30, must rotate by only a small angle A so as to allow the overlappingof the two angular sectors θ_(E) and θ_(R) covered by the two antennae.However, this condition is not sufficient since the observed region ofterrain turns with the reception antenna. In order for all of theterrain to be observed, that is to say in order not to have a loss ofmore than 6 dB (3 dB on transmission and 3 dB at reception) in the echoreceived at the maximum range dmax, it is necessary for the angle A tobe at most equal

In a continuous transmission frequency modulation (CTFM) sonar, theimage rate is high but the resolution in distance is often low since itis inversely proportional to the number of spectral analysis filterspresent in the reception system. Moreover, the low reception band, afterspectral analysis, renders the targets fluctuating, this beingprejudicial to a good probability of detection.

The U.S. Pat. No. 2,871,459 relates to a system intended to explore anannular region of terrain, and which uses several frequencies in orderto avoid coupling between transmission and reception and reverberationphenomena. This system works in continuous transmission mode hencewithout resolution in distance other than the width of the ring, anddoes not make it possible to have a high image rate.

In order to overcome these disadvantages and obtain a sonar which issimple, cheap, has good resolution in distance, has wide-sectortransmission, and offers an image rate increased by a factor n, theinvention proposes another method, to be implemented in a sonar having amaximum range dmax and comprising a movable transmission antennacovering a current sector of angular width θ_(E) and a movable receptionantenna covering a current sector of angular width θ_(R) centred inrelation to an axis turning like the reception antenna, called thereception antenna axis, characterized in that it consists:

in transmitting, in the form of pulses, n uncorrelated successive codes(C1 to Cn) in a current sector of angular width θ_(E) exactly equal tonθ_(R), the transmission and reception antennae continuing to turnduring this time in such a way that the rate of transmission of thecodes corresponds to the time taken by the reception antenna to passfrom the current sector of angular width θ_(R) to a following sector ofangular width θ_(R), in the direction of turning of the receptionantenna;

and in receiving, in the current sector of angular width θ_(R), theechoes of these n codes, coming from n propagation regions which areadjacent in relation to the reception antenna axis, and lying between 0and dmax in space, each of them having a depth equal to dmax/n.

The subject of the invention is also a sonar for the implementation ofthis method. at the start and at the end of rotation of the antennae,the observation sector exhibits "holes", 40, and its edges are ofstaircase shape. These "holes" correspond to the time necessary for thecode signal C₁ and for the signal corresponding to the last codetransmitted, to reach the distance dmax. Account must be taken thereofby making provision for a larger rotation relative to the desired sectorof observation.

This method is applicable to any sonar system whatever its essentialcharacteristics, such as its frequency, its range and its resolution.

The frequency coding can be arbitrary; in practice, for reasons ofsimplicity, a code of pure frequencies will be chosen so that the sonarretains its pulsatile characteristic and hence its good resolution indistance (which is not the case for CTFM devices).

The complexity of the sonar is increased very little, the antennaeremain identical, and a single reception channel is retained, the signalhaving simply to be filtered as a function of the antenna position,hence of time.

Furthermore, as for sonars with preformed channels, it is necessary tofill a wider sector with sound, namely nθ_(R) exactly.

As for the other methods, the transmission and reception antennae can becollinear or partly common.

FIG. 4 represents the region of terrain observed by a side-scan sonarwith parallel preformed channels when the transmissions are codedaccording to the invention.

In underwater acoustics, it is known to use a side-scan sonar withparallel preformed channels, fitted on a towed fish, in order to obtainan image of the sea bottom. In this case, there is no rotation of theantennae over time, but this rotation is replaced by the motion of thecarrier. The invention therefore makes it possible to increase the speedof the carrier and the image rate, or to form only a single channelwhilst maintaining the same image rate.

To obtain a complete sector of observation in the case of the side-scansonar, it is simply necessary to prolong the observation time by 3dmax/4 C, this corresponding to the acquisition phase.

A sonar intended to equip a low-cost vehicle, although powerful, mustalso be low cost; the choice then rests with a single-beam sonar withmechanical scanning, since the formation of channels is alwaysexpensive, given that it necessitates a large number of systems forparallel reception.

By way of example, the characteristics of an embodiment of the sonaraccording to the invention have been chosen such that the range is 75 m,the resolution in bearing is 1° 5, the observation sector is 60°, theobservation time for the sector is 1 s, and the resolution in distanceis 0.2 m. For the embodiment, using the well-known rules of the priorart for calculating acoustic systems, an operating frequency of 750 kHz,a pulse duration of 250 μs, a reception band of 4 kHz, and a mechanicalscanning over 60° have been chosen.

With single-channel use requiring 4s to generate one image, the methodaccording to the invention has been applied by choosing n=4. In thiscase, the speed of rotation of the antenna is 60°/s.

To produce such a sonar, a simple solution consists in choosing a codeof distinct pure frequencies, namely four frequencies, in the receptionband of the sonar. With the current technology in acoustics, a sonar canoperate in a frequency band equal to 40% of the operating frequency,namely in a band equal to 300 kHz for the operating frequency which ischosen equal to 750 kHz. In fact, in this illustrative embodiment, onlya quarter of this possible band is used, namely 75 kHz, the receptionbands being 25 kHz distant. However, it is possible to increase thenumber of frequencies of this sonar as far as n=16, that is to say tomake in a single channel the equivalent of a sonar with 16 preformedchannels.

FIG. 5 represents an example of the distribution in time of thetransmission frequencies.

The frequencies chosen are as follows: 700 kHz, 725 kHz, 750 kHz, and775 kHz. This choice is made asymmetrically to avoid excessively highfrequencies of which the absorption is large. One transmission istriggered every 25milliseconds, namely the equivalent of 18.75 m overthe terrain.

The general overview of this sonar, represented in FIG. 6, comprises atransmission system connected to the transmission antenna, 20, andreception system connected to the reception antenna, 30. The twoantennae are represented as collinear but they can be separated. Theyare constituted by acoustic transducers which, at transmission,transform the electrical signal received into an acoustic wave, orwhich, at reception, transform the acoustic wave into an electricalsignal.

The transmission antenna, 20, comprises a single acoustic transducer 1,serving at one and the same time for transmission and for reception, butone part only of the transducer, 1, will be used on transmission tocover an angular sector equal to 6°, namely the equivalent of 4channels. The directivity of the antenna on transmission will in fact beonly 12° for reasons of symmetry and in order to be able to explorespace in both directions.

A motor, 2, provided with its control electronics, 3, drives theantennae over 60°.

The transmission system comprises:

a frequency generator, 4, which formulates the frequencies of 700 kHz,725 kHz, 750kHz, and 775kHz.

a signal chopper, 5, which forms the pulses to be transmitted. So as toavoid the transmitted pulses perturbing the reception sectors, eachpulse is weighted in time in order to generate secondary frequency lobeswhich are the smallest possible. The weighting used is Gauss or Hamming,or other known weightings;

a power amplifier, 6, which conveys the electronic energy to thetransmitting part of the antenna;

a sequencer, 7, triggers the sequences at the right time and ensures thesynchronising of the whole sonar.

The reception system comprises:

reception transducers forming the reception antenna and to which isconnected a linear preamplifier, 8, which simultaneously receives the 4frequencies and which possesses a filter allowing these 4 frequencies topass, and rejects all the frequencies which do not lie between 698 kHzand 777 kHz. This preamplifier possesses an inhibiting signal whichfunctions when transmission occurs. There will therefore be an absenceof reception during the 4 transmissions, namely during a very short time(4×250 μs), representing 1% of the time. This inhibition is necessarysince there is a large difference in amplitude between the transmittedsignal and the received signal (from 120 dB to 150 dB) and it is notpossible to receive during transmission. This preamplifier 8 possesses avariable gain with time in order to compensate the losses by absorption.If absence of reception during transmissions is unacceptable, it ispossible to double the number of transmitted codes n and change thetransmission synchronisation, displacing it by a half-sequence, namelyby 12.5 ms, every n transmissions, and to thus reconstruct one imagewithout inhibition every 2 n sequences. In this case, the antenna mustrotate twice as fast and only one image out of two is shown complete;

four filters, 9 to 12, allowing real-time reconstruction of the completeimage over the 75 m of propagation. Thus, for example, the first filterwill restore the signal between 0 and 18.75 m, the second between 18.75m and 37.5 m, the third between 37.5 m and 56.25 m and the fourthbetween 56.25m and 75 m. Particular attention will be paid to thequality of these filters which must possess a large dynamic range, ofthe order of 80 dB, and a capacity for rejecting neighbouringfrequencies of the order of 60 dB. In the current state of the art,quartz filters are very well suited to this problem;

finally, the signals emanating from the four filters undergoconventional processing carried out in a processing circuit, 13, such asa precise check of the amplitude of the signals by an amplituderegulator, detection, integration in order to eliminate rapidfluctuations in the signal and improve the signal-to-noise ratio, anddigitising in order to convert the analog signal into a digital signal.

The signals emanating from this processing circuit, 13, are then shownon a viewing device.

The system described in this example reflects a conventional analogembodiment of the reception part. It is equally possible to use ananalog/digital conversion system at the output of the preamplifier, 8,with filtering and digital processing.

Furthermore, a change of frequency or an arbitrary demodulation can alsobe envisaged. Processing operations equivalent to the one describedspecifically, do not call into question the application of the method.

Finally, the invention is not limited to a code of pure frequencies; itis possible to use other codes, for example a code of disjoint modulatedfrequencies.

I claim:
 1. Method for increasing by a factor n the image rate of asonar having a maximum range dmax and comprising a movable transmissionantenna covering a current sector of angular width θ_(E) and a movablereception antenna covering a current sector of angular width θ_(R)centred in relation to an axis turning like the reception antenna,called the reception antenna axis, characterized in that it consists:intransmitting, in the form of pulses, n uncorrelated successive codes (C1to Cn) in a current sector of angular width θ_(E) exactly equal tonθ_(R), the transmission and reception antennae continuing to turnduring this time in such a way that the rate of transmission of thecodes corresponds to the time taken by the reception antenna to passfrom the current sector of angular width θ_(R) to a following sector ofangular width θ_(R), in the direction of turning of the receptionantenna; and in receiving, in the current sector of angular width θ_(R),the echoes of these n codes, coming from n propagation regions which areadjacent in relation to the reception antenna axis, and lying between 0and dmax in space, each of them having a depth equal to dmax/n. 2.Method according to claim 1, characterized in that the successive codesare distinct pure frequencies.
 3. Pulse-action single-beam sonar for theimplementation of the method according to any one of the precedingclaims, comprising a transmission antenna (20) and a reception antenna(30), a transmission system connected to the transmission antenna (20),a reception system connected to the reception antenna (30), thereception system comprising in series a preamplifier (8), filters (9 to12) and a circuit (13) for processing the output signal from thefilters, characterized in that:the transmission antenna is movable andcovers a current sector of angular width θ_(E) the reception antenna ismovable and covers a current sector of angular width θ_(R), θ_(R) beingn times smaller than θ_(E) the transmission system comprises in series afrequency generator (4) controlled by a sequencer (7) for formulating nfrequency codes, and a signal chopper (5) controlled by the sequencer(7) for transmitting, in the form of pulses, the n frequency codes at arate corresponding to the time taken by the reception antenna to passfrom a current sector of angular width θ_(R) to a following sector ofangular width θ_(R), in the direction of turning of the receptionantenna and in that, in the reception system, the filters (9 to 12) arecentred respectively on the frequencies corresponding to the codes inorder to receive, in the current sector of angular width θ_(R), theechoes of the n codes coming from n propagation regions which areadjacent in relation to the reception antenna axis, and lying between 0and dmax in space, each of them having a depth equal to dmax/n, and inorder to reconstruct in real time the complete image of thecorresponding sector of observation θ_(R) over the propagation lengthdmax.
 4. Sonar according to claim 3, characterized in that thepreamplifier (8) possesses an inhibiting signal which functions whentransmission occurs.
 5. Sonar according to claim 3, characterized inthat the filters (9) to (12) are quartz filters.
 6. Sonar according toany one of claims 3 to 5, characterized in that the transmission andreception antennae are collinear or partly common.
 7. Sonar according toany one of claims 3 to 5, characterized in that it is a frontalsingle-beam sonar, the movable transmission and reception antennae beingrotatable.
 8. Sonar according to claim 7, characterized in that it is apulse-action, mechanical scanning and wide-sector multifrequencytransmission sonar.
 9. Sonar according to any one of claims 3 to 5,characterized in that it is a parallel-channel side-scan sonar, thetransmission and reception antennae being movable on account of themotion of the carrier.